Electrically heated bedcover overtemperature control



6, 1968 c. F. JACOBSON 3,396,265

ELECTRICALLY HEATED BEDCOVER OVERTEMPERATURE CONTROL Filed Dec. 30, 1965 Uni ted States Patent 3,396,265 ELECTRICALLY HEATED BEDCOVER OVERTEMPERATURE CONTROL Chester F. Jacobson, Asheboro, N.C., assignor to General Electric Company, a corporation of New York Filed Dec. 30, 1965, Ser. No. 517,570 7 Claims. (Cl. 219212) ABSTRACT OF THE DISCLOSURE and this layer is connected in shunt with one of the heaters, so that the power switch opens whenever such elevated temperatures exist.

The present invention relates to controls and, more particularly, to controls which are used in electrically heated bedcover overtemperature control systems.

It has been known, in electrically heated bedcover control systems, to utilize a temperature sensor material separating a pair of electrical conductors which are distributed throughout the bedcover. The sensor material has a negative temperature coefficient of impedance and is an insulator at normal operating conditions of the bedcover and a conductor of current of control magnitude on the occurrence of an overtemperature condition in the bedcover. The conductors are so arranged in a control circuit with a relay that the relay is actuated by being shunted by current flow through the sensor material when an overtemperature condition is sensed, and this relay actuation opens a circuit to the blanket heater. Even at overtemperature conditions, however, the electrical impedance of the sensor material is still quite high; consequently, the control current flowing therethrough is of a small magnitude.

Effective utilization of this small current in the past has occasioned the use of relatively expensive relays, generally of the electromagnetic type. In the design of an effective relay, problems other than expense are encountered in that, while the relay must be sensitive to operate on currents of small magnitude, iti s desirable that normal fluctuations in line voltage, occurring not as a result of overtemperature in the bedcover, do not acect operation of the relay.

It is therefore an object of the present invention to provide a ctintrol which is rugged in construction, relatively inexpensive to manufacture, and operates effectively with electric current of small magnitude.

A further object of this invention is the provision of a control of the character indicated which provides accurate circuit control and wherein normal fluctuations in supply line voltage do not affect proper operation of the control.

In carrying oct my invention in one form, I provide a control which includes a pair of thermally responsive elements each of which carries an electrical contact. Auxiliary heating means are mounted in intimate heat transfer relation with the elements to cause actuation thereof in accordance with current flow through a temperature sensor material in an electrically heated bedcover control system. The contacts are separated from each other when the bedcover is turned 0 and also when an overtemperature condition occurs in any portion of the bedcover. In normal operation of the bedcover, however, a predetermined dif- 3,396,265 Patented Aug. 6, 1968 ference in heat supplied to the two thermally responsive elements causes the contacts to be engaged with each other. To increase the effect of this heat differential with respect to a given amount of heat input, I provide a heat dissipating member in intimate heat transfer relationship with one of the thermally responsive elements.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. My invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIGURE 1 is a perspective view of a control box cut away to illustrate a preferred control relay embodied in the present invention;

FIGURE 2 is an electrical schematic diagram of a control system using the relay illustrated at FIGURE 1;

FIGURE 3 is an exploded perspective view of the relay illustrated at FIGURE 1; and

FIGURE 4 is a view along line 44 of FIGURE 3.

The specific electrical circuit illustrated by FIGURE 2 and described herein is not my invention, but forms the subject matter of the co-pending application of William D. Ryckman, Ser. No. 517,598, filed concurrently herewith and assigned to the General Electric Company, assignee of this application. The invention herein is directed particularly to an improvement of the thermal switch of the control system.

Now referring more particularly to FIGURES 1 and 3, there is illustrated a relay 10 mounted in a control box 11. The relay comprises a pair of bimetallic blades 12 and 13 which carry switch contacts 14 and 15. The switch contacts are secured near the ends 16 and 17 of the blades, and these ends are free to move under the influence of heat supplied to the blades The opposite ends 18 and 19 of the blades are fixed with respect to each other through the medium of a mounting bracket 20 which is also used to mount the relay in the control box.

The mounting bracket includes a first member 21 which is stamped from steel or other material having good thermal transfer properties. The end portion 19 of blade 13 is secured in intimate heat transfer relationship with the first member such as with rivets 22; and, because the member 21 has good thermal transfer properties, it functions as a heat dissipating member for bimetallic blade 13, as will be hereinafter referred to in greater detail. The mounting bracket further includes a second member 23 which may be formed of the same material as the first member. The blade 12 is separated from the second member 23 through the medium of a block 24 of thermally and electrically insulating plastic material, and rivets 25 are used to secure the blade to the block. Other rivets 2511 are used to secure the block 24 to the second member 23.

In order to supply auxiliary heat to the blades to operate the relay, electrical heating resistors 26 and 27 are mounted in intimate thermal transfer relations-hip with the blades. These resistors may be made of a molded composition resistance material. To assure good thermal transfer between the resistors and the blades, mounting channels 28 and 29 are formed in the blades. These channels are formed by providing spaced slits in the blades and then deforming portions of the blades on opposite sides of the slits alternately in opposite directions from the plane of the blades.

As illustrated clearly at FIG. 3, the insulating block 24 includes a pair of arcuate projections 30 along the bottom surface thereof. These projections couple with grooves 31 formed in side walls of the first bracket mmeber 21 to provide means to initially pivot the bimetallic blade 12 with respect to bimetallic blade 13 for calibration purposes. Thus, the assembly including the second member 23, blade 12, and insulating block 24 is mounted with respect to the first member 21 and blade 13 so that the projections 30 are in the grooves 31. The blade 12 is then pivoted with respect to the blade 13 to achieve a desired gap between the two contacts 14 and 15. Then, the members 21 and 23 are secured together such as by welding, and the required gap is thereby fixed. The rear end of the insulating block is provided with grooves 32 which accommodate lead wires.

The bimetallic blades are mounted in a manner to tolerate normal changes in ambient temperature without substantially changing the initial calibrated gap between the contacts. This may be accomplished by having surfaces 33 and 34 formed of the metal having a low coeflicient of thermal expansion, while the surfaces 35 and 36 (FIG. 4) are formed of the metal having a high coeflicient of thermal expansion. This temperature change toleration may be further assured by making the two bimetallic strips substantially identical to each other.

As conducive to a clearer understanding of the manner in which the relay operates, FIGURE 2 schematically illustrates an electric bed-cover control circuit utilizing this relay. The operation of the system is as follows, assuming that power plug 37 has been plugged into an ordinary alternating current household outlet, and assuming that a female plug 38 extending from the control box is plugged into a male plug 39 extending from the bedcover 40, and also assuming that the blanket is cold: Manual switch 41 is closed and electric current follows a path from power lead 42 through heating resistor 27, conductor 43, heating resistor 26, and conductor 44 to power lead 45 on the other side of the line. The resistance value of resistor 26 is greater than resistance 27; furthermore, bimetallic blade 13 loses heat faster than bimetallic blade 12 because of the heat dissipating number 21. Therefore, more heat is experienced by blade 12 than blade 13, and this causes bimetallic blade 12 to deflect a greater distance than blade 13 to close the initial gap between contacts 14 and 15. Current can now flow through heating wire 46 in the bedcover, assuming also that the contacts 47 and 48 of ambient responsive control 49 are engaged.

The ambient responsive control includes a bimetallic blade 50 which is adapted to be heated by heating resistor 51. When the blade 50 is heated, the contact 48 deflects to the left (as viewed at FIGURE 2) to open the circuit to the blanket heating wire 46. Now, the blade is cooled by the ambient temperature which once again causes the contacts 47 and 48 to be engaged, and this cycle continues to repeat itself. The period of time that the contacts are engaged (hence, the percent on time of the blanket) is therefore determined in part by the ambient temperature and in part by the setting of a control knob 52 by the user.

In order to sense an overtemperature condition in the blanket, a layer 53 of temperature sensing material having a negative temperature coeflicient of impedance is located between the conductors 43 and 44, and the conductors with the temperature sensing material therebetween are disposed throughout the bedcover 40. The layer has a uniform Wall thickness between the conductors of approximately .010 inch. The material 53 may have an impedance on the order of 300,000 to 400,000 ohms between the conductors when it is at room temperature and so, for present purposes it is considered to be :an insulator. As the blanket heats up to normal operating temperatures at the blanket heater 46 of say 120 F., the impedance of layer 53 decreases to approximately 150,000 ohms. Even at this latter value of impedance, the layer 53 is still considered to be an insulator because not enough current can flow through the sensor material between the conductors 43 and 44 to operate the relay 10. If, however, an overtemperature condition, which may be caused by inadvertent bunching or folding, exists in the blanket, the temperature sensed by the layer of material 53 increases even more. Before the temperature in any portion of the bedcover reaches dan- 4 gerous proportions, the impedance of the layer 53 drops to about 50,000 to 60,000 ohms. At the latter value of impedance the sensor material is considered to be a condoctor because it conducts current of control magnitude in the sense that it shunts enough current from resistor 26 to operate the relay. Thus, as is clear from FIGURE 2, the sensor layer 53 is in shunt only with the heater 26 and not with heater 27. In other words, the current through resistor 27 increases so that bimetal 13 is now gaining heat, and the other bimetal 12 is losing heat, thereby causing the bimetallic blades to move away from each other, and the contacts are disengaged toopen the circuit to the electric heating wire 46. After the contacts are open, the bedcover cools. When the bedcover has cooled sufficiently to increase the impedance of layer 53 to values where the layer functions as an insulator, the contacts 14 and 15 will again be engaged. The cycle repeats itself until the cause of the overtemperature condition has been removed.

A suitable material for use in the layer 53 is plasticized polyvinyl chloride having, as an additive, approximately 0.75% stearyldimethylbenzylamrnonium chloride. Other suitable materials are disclosed in US. Patent No. 2,846,560, Jacoby et al., issued on Aug. 5, 1958 and assigned to General Electric Company, assignee of the present application.

In the selection of a proper ratio between the resistance heaters 26 and 27 that will effectively operate the relay in the above circuit, there are two main factors that must be taken into consideration. First, it is desired that the difference between the tempertaure experienced by the blade 12 and the temperature experienced by the blade 13 be relatively high at normal blanket operating temperatures, for it is this temperature difference that closes the contacts and keeps the contacts engaged during normal operation of the bedcover. Secondly, it is desired that this temperature difference approach zero to permit the contacts to open when the blanket reaches overternperature conditions anywhere along the temperature sensor corresponding to an impedance in the sensor layer '53 of 50,000 to 60,000 ohms. I have found that a resistance ratio of 1.8 to 1 for heater 26 to heater 27 gives optimum results. In actual selection of resistance values consideration is given to such factors as keeping the heat in the control box to a minimum, and to keeping the current flowing through the conductors 43 and 44 at a minimum to reduce self-heating of the layer 53 by the conductors. To this end, I have chosen a resistance value of approximately 18,000 ohms for the heater 26 and approximately 10,000 ohms for the heater 27 for operation of the system from a commonly available volt power source.

As can be seen from the above, the closing of the contacts of the relay 10 is dependent on a sufficient diiference in heat to the bimetallic blades to close the gap between the contacts. I increase this differential, and thereby re; duce the time it takes for the contacts to close, by using the afore mentioned mounting bracket 21 as a thermal dissipating member to remove heat from the bimetallic blade 13. By use of the member 21, the blade 13 is heated more slowly than blade 12 and therefore the contacts close more rapidly. As an additional advantage, use of the member 21 limits the amount of heat that the blade Will experience should there be a short circuit between conductors 43 and 44.

Compensation for normal fluctuations in line voltage is accomplished due to the manner in which one bimetal follows the other when the heat to both is either increased or decreased. Thus, assuming the contacts 14 and 15 to be closed, a reduction in line voltage will cause less current to flow through resistor 26 which would tend to open the contacts. This voltage reduction is, however, also experienced by the resistor 27 so that its associated bimetal 13 also cools and follows bimetal 12.

It is to be understood, however, that eve-n though there is the voltage compensation as just described, the actual amount of voltage compensation, or insensitivity to fluct'u a-tion in line voltage, is dependent on the initial contact gap. The reason for this is that as the contact gap is increased, it takes a greater temperature difference between the bimetallic blades to close this gap. Even though there is a following action between the blades upon voltage fluctuation, the actual difference in temperature in the blades is decreased as the line voltage drops. This, therefore, would suggest use of a small initial gap. On the other hand, it is the gap that furnishes a good positive contact breaking force when the heat to bimetallic blade 12 has sufficiently decreased in an over-temperature condition of the blanket. The selection of a proper gap is then a compromise between the voltage sensitivity desired, and the contact breaking force required. I have found that a contact gap of approximately .014 inch between the contacts 14 and 15 with no power into the heaters 26 and 27 provides a good con-tact breaking force but still assures satisfactory operation of the relay down to voltage of about 100 volts AC.

It may be desirable to use a neon indicator lamp 54 and associated current limiting resistor 55 to indicate when the switch 41 is closed.

While preferred embodiments of the invention have been shown and described, various other embodiments and modifications thereof will be apparent to those skilled in the art, but will fall within the spirit and scope of invention as defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A control for an electrically heated bedcover or the like comprising:

(a) an electric heating circuit including heating means therein;

('b) first and second thermally responsive elements;

(c) first and second electrical heaters in heat transfer relationship respectively with said first and second thermally responsive elements;

((1) first and second normally open switch contacts in said heating circuit and movable into engagement with each other by said thermally responsive ele ments when said second heater supplies a predetermined greater amount of heat to said second element than said first heater supplies to said first element;

(e) a temperature sensor distributed over the area of said heating means and including a pair of conductors separated by :a layer of material having a negative temperature coefficient of impedance;

(f) means for connecting said two heaters across a power line in electrical series with each other and in electrical series with said pair of temperature sensor conductors and with said overtemperature sensor layer in shunt only with the second of said two heaters; and

(g) a heat dissipating member in intimate heat transfer relationship with said first thermally responsive element for increasing the heat differential between said thermally responsive elements when said elements are heated by said heaters.

2. The control as set forth in claim 1 including a thermally conducting mounting bracket for mounting said thermally responsive elements in a control box, and wherein said second thermally responsive element is thermally insulated from said mounting bracket.

3. The control as set forth in claim 1 wherein said thermally responsive elements comprise bimetallic blades mounted in face to face relationship for movement in the same direction under influence of heat.

4. The control as set forth in claim 3 wherein said heaters are formed of a molded composition resistance material and mounted in channels formed in said blades, and wherein the resistance value of said second heater is approximately 1.8 times the resistance value of said first heater 5. The control as set forth in claim 3 wherein said contacts are initially separated from each other by a gap of approximately .014 inch when said heaters are not supplying heat to said blades.

6. A control for an electrically heated bedcover or the like comprising:

(a) an electric heating circuit including heating means therein;

(b) first and second bimetallic blades each having a fixed end and a movable end, said movable ends flexing in the same direction under the influence of heat;

(0) switch contacts in said heating circuit carried by said movable ends of said blades initially separated from each other and for engaging each other only when there is a predetermined difference in heat supplied to said first and second blades;

(d) first and second heaters in intimate heat transfer relation with said blades for supplying heat to said blades;

(e) a mounting bracket of thermally conducting material for mounting said blades in a control case, said mounting bracket including a first member connected to said first blade in intimate thermal transfer relationship therewith, a second member connected to said second blade and separated therefrom by a layer of insulating material, and means for initially pivotally connecting said first and second members to establish a predetermined gap between said contacts prior to rigidly connecting said first and second members; and

(f) means for connecting said heaters to an overtemperature sensor distributed over the bedcover whereby said contacts :are engaged during normal operation of the bedcover and disengaged upon occurrence of an overtemperature condition in said bedcover.

7. A control as set forth in claim 6 wherein said pivotal connecting means includes an arcuate projection on said insulating block received in arcuate grooves in said first member of said bracket.

References Cited UNITED STATES PATENTS 2,510,038 5/1950 Rudahl 219-212 2,846,560 8/1958 Jacoby et al. 219--505 X 2,928,927 3/1960 Taylor 219-494 3,114,820 12/1963 Holmes 2l9494 X 3,213,329 10/1965 Somers 317-132 BERNARD A. GILHEANY, Primary Examiner.

VOLODYMYR Y. MAYEWSKY, Assistant Examiner. 

